For lightweight cars, thin sheet steel with high tensile strength is desired and investigated. However, sheet steel with increased tensile strength often has poor press formability. Therefore, it is desired to develop sheet steel with both good press formability and high tensile strength.
As one type of sheet steel with both good press formability and high tensile strength applicable to the production of car bodies, known is bake-hardenable sheet steel. This is, after having been worked by pressing, painted and baked generally at high temperatures falling between 100 and 200.degree. C., whereby its yield strength is increased due to the action of the solute carbon (C) existing therein. Precisely, the solute C in the sheet steel having been heated for baking is fixed to the dislocation site as introduced into the steel while the steel is worked by pressing, to prevent the dislocation site from being moved in the steel, thereby increasing the yield strength of the steel. It is said that the bake-hardenable sheet steel for cars of that type must have a degree of bake hardenability (BH) of not smaller than 30 MPa.
However, in the bake-hardenable sheet steel, the dislocation site is often partly fixed by the solute C before the steel is worked by pressing, causing ridged surface defects of so-called stretcher strains when the steel is worked by pressing. The stretcher strains result from the increase in the yield point elongation of the steel being press-worked, and greatly lowers the product quality of the steel. Thus, the known, bake-hardenable sheet steel is problematic in that its anti-aging property is poor.
To solve the problem of such poor anti-aging property, proposed is another type of bake-hardenable sheet steel with improved anti-aging property. For example, Japanese Examined Patent Publication (JP-B) No. Sho-61-12008 discloses a method for producing dual-phase structured, high-strength sheet steel for deep drawing, which comprises a step of hot-rolling extra low-C steel containing both Nb of from 2 to 10 times the C content of the steel and B of not smaller than 0.3 times the N content thereof, at a low coiling temperature ranging between 550 and 200.degree. C., as combined with a step of annealing it within the .alpha.-.gamma. dual-phase range of the steel followed by rapidly cooling it, to thereby make the resulting sheet steel have an elevated r value and good bake-hardenability. The disclosed method is characterized by the heating within the .alpha.-.gamma. dual-phase range of the steel followed by the rapid cooling to give the dual-phase structure composed of acicular ferrite and ferrite. The structure contains solute C and has a high degree of bake-hardenability (BH), in which, however, most solute C is trapped by the acicular ferrite having a high dislocation density. Therefore, the yield point elongation of the sheet steel is increased little after annealing.
In addition, the method disclosed in JP-B Sho-61-12008 is problematic in that it requires high-temperature annealing within the .alpha.-.gamma. dual-phase range of extra low-C steel and that the .alpha.-.gamma. dual-phase range of the steel is too narrow to stably ensure the intended quality of the steel throughout the process of the method.
As being essentially used for outer panels of car bodies, bake-hardenable sheet steel discussed herein requires good ductility for uniform elongation relative to its press formability, in particular, to its stretch formability. The ductility for uniform elongation is indicated by the maximum tensile strength of sheet steel tested in a tensile test. It is said that sheet steel having a lower yield strength or having a higher work-hardenability index, n, shall have better ductility for uniform elongation. For bake-hardenable sheet steel, however, that having a higher yield strength after baked is more preferred. Therefore, it has heretofore been difficult to obtain favorable bake-hardenable sheet steel having good ductility for uniform elongation.
The present invention is to solve the problems noted above, and its object is to provide bake-hardenable sheet steel and galvanized sheet steel having good anti-aging property and capable of being stably produced on an industrial scale. The object is also to provide such sheet steel having good ductility for uniform elongation and having improved press formability.
Given the situation, we, the present inventors have assiduously studied in order to obtain extra low-C sheet steel having a high degree of BH and good anti-aging property, and, as a result, have found that, in sheet steel, solute C expressing BH-ability differs from solute C participating in room-temperature aging with respect to the sites where they exist. Specifically, in the bake-hardening treatment for heating sheet steel at a high temperature of 170.degree. C. or so, all solute C existing inside and around the grains of steel, or that is, all intragranular and intergranular solute C in steel participates in the BH-ability of the steel. On the other hand, however, since the temperature for room-temperature aging treatment is lower than that for bake-hardening treatment, the intergranular solute C existing in the grain boundaries of steel could not diffuse into the grains but are still kept stabilized in the grain boundaries during room-temperature aging treatment. As a result, only the intragranular solute C existing inside the grains of steel participates in room-temperature aging of steel, while the intergranular solute C existing around the grains has no influence on the room-temperature treatment. Thus, the intergranular solute C participates in the BH-ability of steel but not in the anti-aging property thereof. As opposed to this, the intragranular solute C participates in both the BH-ability and the anti-aging property of steel.
In addition, the present inventors have further found that sheet steel in which the ratio of the misorientation to the grain size is defined to be not smaller than a specific value may have good anti-aging property even though it has high BH-ability. Specifically, by reducing the grain size while increasing the intergranular area to thereby enlarge the degree of misorientation in sheet steel, the amount of C existing in grain boundaries in the sheet steel can be increased whereby both the BH-ability and the anti-aging property of the sheet steel can be improved. In this connection, still another finding of the inventors is that the reduction in the amount of P that interferes with the intergranular segregation of C in sheet steel is important.
Moreover, the inventors have found that the amount of solute C existing inside and around the grains in sheet steel can be well controlled by optimizing the relationship between the slab reheating temperature and the S content of the steel, whereby the BH-ability and the anti-aging property of the sheet steel can be much improved.
The present invention is based on those findings of the inventors.